function [ ] = mx_eyelink_eye_available( )
%MX_EYELINK_EYE_AVAILABLE After calling eyelink_wait_for_block_start(), or 
% after at least one sample or eye event has been read, can be used to check %
% which eyes data is available for.
%
% Returns:
% One of these constants, defined in EYE_DATA.H: 0 if left eye data. 1 if right
% eye data. 2 if both left and right eye data. -1 if no eye data is available
%
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% Copyright Eduardo Aponte 2013
% aponteeduardo@gmail.com